Comment by ky3

The repo looks to be here:

https://github.com/combinatorylogic/clike

Because you have a functional and also imperative language (pfront?) that your compiler is written in, you avoid memory churn by ensuring "visitor nodes are chained together", is that right?

So pass fusion for compiling clike does away with the destructive matching and rebuilding of untransformed subexpressions by linking pointers back to the original.